Remove support for using the bsd-uthread target on FreeBSD.

Remove support for using the bsd-uthread target on FreeBSD.

The bsd-uthread target supports an old thread library ("libc_r") that
was last included in FreeBSD release 6.4 released in 2008.  However,
this library has not been used as the default library since FreeBSD
5.0 (2003) and 4.11 (2005).  Thread support for modern FreeBSD binaries
is provided via "normal" LWP support in core files and the native
FreeBSD target.

gdb/ChangeLog:

	* amd64-fbsd-tdep.c: Remove "bsd-uthread.h" include.
	(amd64fbsd_jmp_buf_reg_offset): Remove.
	(amd64fbsd_supply_uthread): Remove function.
	(amd64fbsd_collect_uthread): Remove function.
	(amd64fbsd_init_abi): Don't set bsd-uthread callbacks.
	* configure.tgt (i[34567]86-*-freebsd*): Remove bsd-uthread.o.
	(x86_64-*-freebsd*): Remove bsd-uthread.o.
	(fbsd-nat.c): Update comment.
	* i386-fbsd-tdep.c: Remove "bsd-uthread.h" include.
	(i386fbsd_jmp_buf_reg_offset): Remove.
	(i386fbsd_supply_uthread): Remove function.
	(i386fbsd_collect_uthread): Remove function.
	(i386fbsd_init_abi): Don't set bsd-uthread callbacks.